Sphingolipids are being increasingly recognized as key mediators of inflammatory cascades of critical importance in multiple diseases, including rheumatoid arthritis and osteoarthritis. Most importantly, sphingosine 1-phosphate produced by sphingosine kinase (SK) mediates the effects of tumor necrosis factor-1 and interleukin-12, pro-inflammatory cytokines that are central to pathologic processes in these diseases. Because of this pivotal role of SK in regulating inflammation, we are developing SK inhibitors to be used as drugs to treat arthritis. In Phase I of this project, we demonstrated that our proprietary SK inhibitors block signaling pathways induced by inflammatory cytokines in cell culture. Importantly, we further demonstrated that oral administration of the SK inhibitors attenuates the development of collagen-induced arthritis in mice and adjuvant-induced arthritis in rats. These therapeutic effects of the SK inhibitors are manifested without toxicity to the animals. These studies provide the first proof-of-principle demonstration that SK inhibitors are likely to be effective in the treatment of arthritis. In Phase II of this project, we will more thoroughly evaluate the therapeutic efficacy of an optimized formulation of the leading SK inhibitor in multiple models of rheumatoid arthritis and osteoarthritis. Additionally, we will determine the optimal schedule for treatment of the animals, and examine the therapeutic effects of combining the SK inhibitor with established anti-arthritis drugs. Both of these activities will be important for subsequent translation to clinical trials. The following Specific Aims will be addressed: 1. To confirm the therapeutic activity of ABC294640 in three experimental models of rheumatoid arthritis. 2. To determine the therapeutic activity of ABC294640 in two experimental models of osteoarthritis. 3. To determine the therapeutic activity of ABC294640 in combination with established anti- arthritis drugs. 4. To determine the pharmacokinetics and the optimal treatment schedule for ABC294640. Upon completion of these experiments, we will be ready to begin clinical testing of a new chemical entity directed toward a new molecular target critical for progression of arthritis. Therefore, these studies provide a focused approach for moving a novel inhibitor of SK into clinical trials for the treatment of these debilitating diseases. PUBLIC HEALTH RELEVANCE Sphingosine kinase (SK) is a critical enzyme involved in the pathogenesis of arthritis. In this project, we have developed inhibitors of SK and shown them to have therapeutic efficacy in arthritis models. Continued development of these compounds may provide important new drugs for the treatment of arthritis.